The present invention relates to a bearing system and, more particulary, to a bearing system for high speed spindles of grinding machines wherein at least one front bearing located close to flange of the spindle and a rear bearing are each provided with at least three support rolls acting equal angularly on the high speed spindle, and wherein the at least three support rolls having a diameter larger than a diameter of the spindle.
High demands as regards the operating speed are made on spindles or driving shafts of modern machine tools such as. In the case of rotating tools, for example grinding wheels, this calls for high speeds on the tool or the spindle driving the same. Thus, with spindles having speeds in excess of 100,000 rpm, conventional antifriction or slide bearings are not practical. Moreover, as a result of the extreme centrifugal forces, it is not possible to obtain the necessary rigidity for precise machining in the bearing system.
Therefore numerous attempts have been made to increase this rigidity by moving away from conventional bearing technology, in that, for example the spindles are received between larger diameter support rolls, which are in turn mounted as planetary rotating rolls in a stationary outer ring or race of the spindle casing. As a result of the larger diameter of the rotating support rolls a speed reduction takes place, so that at least theoretically it is possible to better control the centrifugal forces in the bearing system. However, in practice this attempt has failed, because the extremely high precision demands made on machine tools of this type cannot be satisfied. As a result of the many bearing surfaces in different spatial arrangement, namely spindle/support roll and support roll/outer ring, this bearing system, even in the case of the most precise manufacture and fitting of the bearing parts, necessarily has clearances and is consequently not sufficiently rigid to permit precise machining.
The problem of the present invention is to provide a bearing system which, at speeds up to 150,000 rpm, still has an adequate rigidity to permit precise machining, even at such high speeds.
On the basis of the aforementioned bearing system, this problem is inventively solved in that the support rolls are fixed and are mounted with roller journals provided on either side in small diameter precision bearings in the casing receiving the spindles and the support rolls.
Through the reception of the spindles between larger support rolls, in the invention there is also initially a speed reduction on the support rolls in the ratio of spindle diameter to support roll diameter. This lower speed can be absorbed in relatively problem free manner in small precision ballbearings with for example, a bore diameter of 15 mm, as a result of the fixed arrangement of the support rolls. Due to the fixed arrangement of the support rolls, the bearing system has a high rigidity and the narrow tolerances required within the complete bearing system necessary for the machining accuracy can be achieved.
In order to exclude any clearance, even after prolonged operation, according to a preferred embodiment, the spindle has conical portions as bearing surfaces which are inclined towards one another, and the support rolls are arranged with their axis parallel to the circumferential surface of the conical portions, and at least the support rolls of one bearing are adjustable axially parallel to the spindle.
The above features make it possible to prestress the bearing system, in that the support rolls of at least one bearing are adjustable. The support rolls of at least one bearing can be individually and/or jointly adjustable axially parallel to the spindle.
As a result of the adjustment of individual support rolls and all the support rolls together, the bearings can be prestressed in the axial direction of the spindle such that for example a clearance-free bearing system can be obtained and also maintained during long operating periods.
For the prestressing of individual rolls or for the joint adjustment of all the rolls, it is advantageous to provide a pressure medium, hydraulic or pneumatic, or a spring tension, while a common adjustment of all the support rolls of a bearing can also take place by a servomotor.
The adjustment of individual rolls also offers the possibility of inclining the spindles under clearly defined angles, so that the conicities can be ground with a small cone angle or the grinding wheel can be adjusted with respect to the workpiece and, consequently, a clearance angle can be formed on the tool.
Preferably prestressing exclusively takes place on the support rolls of the rear bearing, which is further from the spindle flange with the tool.
In order to ensure a correct running of the support rolls, the bearing surface thereof is preferably domed, so that at least theoretically there is only a point contact of the support roll on the conical portion of the spindle.
According to a further development of the invention the spindle is driven by means of a HF-motor mounted thereon and the bearings are located on either side of the HF motor. This construction leads to a short, compact bearing system.